Revert "Revert 249676 "Have the unload event execute in background on cr...""

This fixes a bug in the original CL which may have caused a crash in
ProfileDestroyer.

BUG=343002,323528,342361

Review URL: https://codereview.chromium.org/180993003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254007 0039d316-1c4b-4281-b951-d872f2087c98
Adds field trial support for disabling ignore autocomplete='off'.

Ignore autocomplete='off' is now the default in Chrome. There is a flag for
users to disable it if they want. This adds support for a field trial to disable
this feature if something goes wrong.

BUG=346708

Review URL: https://codereview.chromium.org/182543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254008 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254008